The potential for aircraft collisions was recognized early in the aviation age. Initially, collision avoidance was based on the pilot's visual sense; the pilot was expected to visually identify collision threats and avoid them. Since at least the 1950's, research has been conducted to enhance collision avoidance techniques, although "see and avoid" remains the basic technique and other techniques are supplementary. Almost exclusively, those enhancements have been based on radio frequency or optical transmissions.
Since then, the government has developed and implemented the Air Traffic Control Radar Beacon System (ATCRBS). This system comprises ground based and airborne carried equipment. The ground based equipment includes two different types of radar emitters located at each of a plurality of Air Traffic Control (ATC) stations. The first type of radar equipment is referred to as the Primary Surveillance Radar (PSR), or sometimes simply as the primary radar. The primary radar operates by sending out microwave energy which is reflected by the metal surfaces of aircraft skin producing a reflected signal that is received back at the radar site and displayed for locating and displaying the position of aircraft to an air traffic controller. The second type of radar, referred to as the Secondary Surveillance Radar (SSR), or simply secondary radar, is unlike the primary radar in that it does not depend on reflected signals. The secondary radar transmits a coded 1030 MHz microwave interrogation signal. Airborne equipment includes a transponder which receive these signals, interprets them, and transmits in turn a reply back to the SSR site. The reply is transmitted back to the ground on a microwave frequency of 1090 MHz. Thus the combined ATCRBS includes ground sites comprising primary surveillance and secondary surveillance radar and aircraft carried transponders.